Background: Sarcopenia is characterized by loss of muscle mass and weakness, leading to frailty, and impaired mobility. Prolonged muscle disuse, as occurs during hospitalization and bed rest, can accelerate the progression sarcopenia. Sarcopenia and its related co-morbidities are an enormous public health problem in the U.S. However, the mechanisms underlying sarcopenia have not been elucidated. Elevated muscle oxidative stress inhibits protein synthesis and increases protein breakdown, and has been identified as a mediator of muscle atrophy during disuse. While the source of oxidative stress during muscle atrophy has not been identified, reactive oxygen species (ROS) produced by mitochondria may play a role. The regulators of ROS production during prolonged muscle inactivity remains largely unknown. However, evidence suggests that the muscle lipid ceramide may play a role. Aim: The aim of this study is to determine the role of ceramide in mitochondrial ROS production and muscle atrophy in the context of aging and disuse. Methods: A hind limb model will be used to induce muscle atrophy in young and old mice. By specifically manipulating either ceramide content (myriocin treatment, DES-1 KO) or H2O2 (mCAT over-expression) within skeletal muscle using different experimental paradigms, we will be able to more carefully discern their roles in disuse atrophy. Muscle performance, including fatigability and maximal strength will be determined, as ceramide has also been shown to mediate fatigue and loss of strength. We will recruit sarcopenic low physically functioning and non-sarcopenic high physically functioning elderly individuals to define the relationships between muscle mass, function and muscle biopsy-derived ceramide species, and mitochondrial bioenergetics. Significance: The proposed studies will provide, for the first time, novel translational evidence i both animals and humans that intramyocellular ceramides contribute to mitochondrial ROS production, sarcopenia and loss of physical function.